The dehydration of sucrose
The dehydration of sucrose by sulfuric acid is an interesting and spectacular demonstration and I have often used it, in my inorganic chemistry lectures, to illustrate the dehydrating and oxidising power of concentrated sulfuric acid. The recent article in the Exhibition chemistry series clearly presents this demonstration and the pictures are excellent.1 Colin Baker states that sulfuric acid dehydrates the carbohydrate to carbon and then oxidises the carbon to CO2, so that both CO2 and SO2 (formed by reduction of H2SO4) are evolved. Therefore, safety notes for SO2 are detailed. Nevertheless, the evolved gases also contain CO and, indeed, the predominant gas is CO, so that safety cautions concerning this gas should be stressed.
Almost 30 years ago, Meeks performed one of the more detailed experiments to investigate the products of the reaction of sucrose with sulfuric acid.2 He found that the volume (and molar) composition of the gas produced was about 66 per cent CO, 17 per cent CO2 and 17 per cent SO2, after condensation of steam. He also found that the carbonaceous mass contained carbonyl groups, indicating that dehydration was not complete. Another comprehensive study of the dehydration of different carbohydrates by sulfuric acid was reported by Dolson et al.3
The reaction of carbon with sulfuric acid in the presence of an oxidant can give rise to graphite intercalation compounds, but sulfuric acid does not oxidise carbon to carbon oxides. Therefore, the formation of CO, CO2 and SO2 is more likely to occur by the direct oxidation of sucrose or some other organic intermediate compound by sulfuric acid, rather than by the oxidation of carbon after the dehydration of sucrose. In any case, the reaction of sucrose with sulfuric acid is complex, but the formation of carbon, CO, CO2 and SO2 clearly illustrates the dehydrating and oxidising power of concentrated sulfuric acid.
Universidad Autónoma de Madrid, Spain
- C. Baker, Educ. Chem., 2007, 44 (2), 42
- E. G. Meeks, Sch. Sci. Rev., 1979, 61, 281
- D. A. Dolson et al, J. Chem. Educ., 1995, 72, 927